The present invention is directed to a fuse and a method for making a fuse. More particularly, the invention relates to a subminiature fuse having a tubular body and a fuse element disposed within the interior of the body.
Manufacturing subminiature fuses is labor and time intensive in handling and assembling the parts and in connecting the fuse element to the end terminations. This is of particular concern in manufacturing subminiature time lag fuses, in which a fuse element is disposed between the end terminations in air or gas.
It is a well known fact that for a fuse to perform consistently on low overloads, the current carrying element has to be kept away from touching the inside walls of the fuse body. Typically, funnels are used to guide and maintain the fuse element away from the inside walls of the fuse body during first end soldering. After first end solder, the funnels are removed for reuse. In a separate step, second end solder joints are made to complete the fuse assembly.
In another variation, end caps with holes are glued at each end of the fuse body and fuse wire is then passed through holes for solder connection at the outer surface of the end caps.
In some constructions, a fuse wire element is placed and attached to a supporting bridge of fiber, glass melamine, ceramic etc. This bridge will either have a metallic foil mechanically attached at each end or a metallized portion to establish a solder joint between the fuse element and the end cap.
All these techniques require many steps in assembly, are slow and therefore very expensive.
The present invention describes a method of metallizing a fuse body so that a meniscus is formed on the inside of the fuse body at each end. This meniscus keeps the element away from contacting the inside wall of the fuse body and thus helps to achieve a consistent performance on low overloads.
Also, the assembly cost is much lower since the process is capable of simultaneous soldering of termination at each end.
The present invention provides a method for manufacturing subminiature fuses that is a low-cost, batch process method. The method is suitable for manufacturing a variety of subminiature fuses, including time-lag fuses, surface mount fuses, leaded fuses, and other types, as will be understood through the following description.
The present invention also provides a subminiature fuse that is capable of withstanding the stresses of circuit board assembly, soldering, and cleaning without degradation of the fuse or its operability. The present invention provides a subminiature fuse with a sealed cavity to contain the fusible element.
According to the invention, an elongated, hollow fuse body, having an internal cavity for containing a fuse element, is coated at end portions with a metallic material to form a metallic coating. Upon assembly of a fuse element, solder preforms, and end terminations with the fuse body, the unit is heated and the metallic coating, the end terminations and fuse element are bonded together forming a seal to close the cavity of the fuse body.
According to one embodiment of the invention, the metallic coating may include a thickened region internally on the fuse body to form a guide for the fuse element and to facilitate attachment of the fuse element to the remainder of the fuse.
According to the invention, the end terminations may comprise end plates formed of electrical conductive material, which are attached to end faces of the fuse body. The end plates are soldered or brazed to the end faces of the body on the metallized coating, which facilitates forming a secure bond. The end plates may be square, rectangular, or circular (i.e., disk shaped). Disk-shaped end plates eliminate the need to orient the end plate to the side edges of the fuse body. The end terminations may alternatively comprise caps which are placed over the metallized end portions of the fuse body. The solder or brazing alloy preforms on the end portions of the fuse body will melt and bond with the end caps or the disk-shaped elements to form the seal of the interior cavity. The end plates or caps may be provided with axially-extending leads, if desired.
A method according to the invention includes the steps of applying metallized coatings to axially opposite surface end portions of a hollow fuse body and placing a fuse element in the internal cavity in the fuse body so that the fuse element extends from the first end to the second end of the cavity. A solder or brazing alloy preform is placed at each of the first and second ends of the cavity and an end termination is placed at each of the first and second ends in contact with the preform. The assembly thus formed is heated to a temperature sufficient to cause the solder preforms to soften and flow for bonding the fuse element to the end terminations. The end terminations then form seals closing the ends of the cavity.
According to another aspect of the invention, the method includes the steps, prior to the heating step, of placing the assembled fuse body, fuse element, solder or brazing preforms and end terminations in an environmentally controlled chamber, and charging the chamber with a selected gas so that the cavity is filled with the selected gas before being sealed.
According to the invention, the selected gas may be at a pressure greater than atmospheric pressure, or alternately at a pressure less than atmospheric pressure.
The selected gas may be an inert gas, such as nitrogen. Alternatively, the gas may be sulfur hexafluoride, which is believed to provide arc suppression, to improve the interrupting ability of the fuse.
According to another aspect of the invention, the fuse body is placed in a vertically-oriented recess in a fixture. The fuse element is then placed in the cavity in the fuse body. The fuse components are heated for bonding in this fixture, which eliminates special handling of the fuse element to achieve a gas or air insulation around the fuse element. According to another aspect of the invention, the fixture may be vibrated in stages to cause the fuse bodies to enter the recesses in the fixture.
The fuse element includes a substantially rigid structure so that it maintains its orientation in the cavity and avoids contact with the inside surface of the fuse body. The fuse element may comprise a wire element wound on an electrically insulating core. Alternatively,the fuse element may comprise an electrically conductive film element carried on an electrically insulating substrate. Other fuse element structures may also be suitable, for example, a metallic link or a wire.